I have a 4 node cluster with 2 nodes assigned to 2 replica groups. When master node of one of my replicas failed, as mentioned in document, I expected my other node (replica) to become master but instead it went into UNKNOWN state. Hence I am unable to write data.

Please see the documentation: http://docs.oracle.com/cd/NOSQL/html/AdminGuide/Oracle-NoSQLDB-Admin.pdf
Search for "replication factor". On page 11, it explains why you should not choose a replication factor of 2 - if a node fails, you will not have majority to elect a new master.

You can either add more hardware (minimum repl factor of 3) or run multiple replication nodes per storage node so that you can get a higher replication factor. Appropriate tuning may be required to get the best performance.